1. Field of the Invention
The present invention relates to a magnetic circuit for a voice coil motor of a flat-coil type or, in particular, to an improvement in a magnetic circuit for a voice coil motor of a flat-coil type used with a magnetic disk device or the like, comprising a rotor having a flat coil for positioning a head at the desired track on the disk and a magnetic circuit constituting a stator.
2. Description of the Related Art
In recent years, magnetic disk devices have remarkably increased in capacity, making it critical to improve the positioning accuracy of the magnetic head. At the same time, demand has increased for a higher access speed and a lower power consumption, thus giving rise to the necessity of a high-efficiency access motor. The improved recording density, on the other hand, has led to the demand for an efficient voice coil motor of a flat-coil type having a flat torque characteristic which can access the desired track at higher speed.
A conventional magnetic disk device has a bathtub-shaped base housing therein at least a disk rotatable by a spindle motor, a rotor carrying a head for writing or reading a signal into or from the disk, and a voice coil motor for driving the rotor. A cover is applied to the opening of the base.
The cover is made of a single plate and has a surface formed with steps by the pressing technique. The rotational shafts of the rotor and the spindle motor are fixed on the inner face of the cover by screws.
The conventional magnetic disk device also includes a depression, having a hole, formed in the base bottom for fixing the spindle motor. The spindle motor is mounted in the depression, and disks 2 are arranged around the spindle motor.
The voice coil motor mounted at a corner of the base includes a magnetic circuit constituting a stator and a rotor. A head for each recording surface on the disks 2 is mounted at an end of the rotor. A flat coil is interposed between two yokes at the other end of the rotor.
The magnetic circuit includes an upper yoke, a lower yoke, two side yokes for coupling the upper and lower yokes at the ends thereof to each other, a boss for supporting the upper yoke at three points, and a magnet mounted on the lower yoke. The upper and lower yokes are arranged in parallel to each other. The upper and lower yokes are adapted to be placed in an opposed relationship to the flat coil at the above-mentioned other end of the rotor when the voice coil motor is incorporated in the base. A large-diameter portion formed at the central portion of each side yoke functions as a stopper for defining the maximum rotational angle of the rotor.
The magnet is made of a curved flat plate defined by an inner peripheral surface and an outer peripheral surface concentric about the rotational center of the rotor and two radial surfaces. The width between the inner peripheral surface and the outer peripheral surface of the magnet is substantially fixed except for the ends thereof. The magnet is divided into a S pole and a N pole located on the two sides, respectively, of a line passing through the rotational center of the rotor.
The rotor is adapted to swing between the two side yokes with the maximum yawing angle thereof defined by the latter.
The above-mentioned magnet made of a curved flat plate, however, poses the problem that the amount of magnetic flux received, from the magnet, by the flat coil for swinging the rotor is reduced at the ends of the magnet to such an extent that the torque constant curve of the magnetic circuit is not flat with respect to the rotor yawing angle. Specifically, assuming that the yawing angle is zero when the rotor is located at the center of the magnetic circuit, the torque constant sharply declines with the increase in the rotor yawing angle. As a result, different amounts of current are required for the flat coil to swing the rotor by the same angle between when the rotor yawing angle is small and when the rotor yawing angle is large, thereby making it difficult to control the rotation of the rotor.
In view of this, an attempt has been made to flatten the torque constant curve by forming at least a recess by cutting off at least a portion of the outer peripheral surface of the magnet.
In the conventional magnetic circuit for a voice coil motor of flat-coil type with the torque constant curve thereof flattened by forming at least a recess in at least a portion of the outer peripheral surface of the magnet, however, the torque constant that can be actually secured is reduced so that the torque for swinging the rotor is considerably decreased, resulting in a lower peak value of the torque constant. Although the torque constant curve is flattened as compared with a magnet having no such a recess in a portion of the outer peripheral surface, the problem of the conventional magnetic circuit is that an increased amount of current must be inconveniently supplied to the flat coil of the rotor.